The present invention relates to a backlighting apparatus such as an illumination light source for a flat panel display, and a display apparatus using the same.
A liquid-crystal display (LCD) apparatus is thin and light and has great utility as low-power-consumption display apparatus. The LCD apparatus are widely used as display devices for notebook-type personal computers, and have recently been used as display devices in some types of office-use desktop type computers.
The LCD display apparatus comprises a plate-like display panel called "flat panel." The LCD display apparatus has advantageous in that compared to a display apparatus using a CRT, the LCD display apparatus has a smaller depth dimension and requires a narrower space for installation.
It is expected that the LCD display apparatus will be used to realize a wall-type large-screen TV for household use. FIG. 1 shows an example of a typical structure of a conventional LCD apparatus.
As is shown in FIG. 1, the LCD apparatus comprises a liquid-crystal (LC) panel 100 and a backlight 200 for illuminating the LC panel 100. The LC panel 100 comprises an LC layer 110, two glass plates 120a and 120b sandwiching the LC panel 100, and two polarizing plates 130a and 130b sandwiching the glass plates 120a and 120b.
The glass plates 120a and 120b are provided with transparent electrodes for driving the liquid crystal and switching elements such as thin-film transistors. The voltage applied to the LC layer 110 is controlled by a drive signal delivered from a drive circuit (not shown).
The polarization state of light passing through the LC layer 110 is varied in accordance with the voltage applied to the LC layer 110. In accordance with the polarization state, the light transmissivity of the LC panel 100 varies and gray-level display is achieved.
The backlight 200 for illuminating the LC panel 100 comprises a fluorescent tube 270, a reflection plate 260 surrounding the fluorescent tube 270 and a light guide plate 240, a reflection plate 250 disposed under the light guide plate 240, a diffusion plate 230 disposed over the light guide plate 240, and two prism sheets 220 and 210 disposed over the diffusion plate 230.
Light emanating from the light source or the fluorescent tube 270 is made incident on the light guide plate 240 directly or after reflected by the reflection plate 260. The light propagates through the light guide plate 240 while repeating total reflection, and part of the light is dispersed by dispersion marks 245 formed on the lower surface of the light guide plate 240.
The dispersed light travels directly or is reflected by the reflection plate 250 to the LC panel, and then it is diffused by the diffusion plate 230. The diffused light travels through the two prism sheets 220 and 210 and illuminates the LC panel 100.
If the dispersion marks 245 are arranged optimally, uniform illumination is achieved. In addition, the directivity of illumination light is improved by the prism sheets 220 and 210, so that the luminance of display, as viewed from the front face of the LC panel on the observer's side (opposite to the side of the backlight 200) is improved.
As has been described above, the thin and light display apparatus is achieved by combining the LC panel 100 and the backlight 200 comprising thin plate-like members. However, compared to the CRT display apparatus, the conventional LCD apparatus are not satisfactory with respect to the screen size and screen luminance. A larger screen and a higher luminance are desired for the LCD apparatus. Besides, the conventional LCD apparatus has a relatively narrow range of viewing zone in which high-quality display is obtained. An enlargement of the viewing zone is also desired.
The enlargement in screen size and the increase in viewing zone are mutually associated objects to be achieved. In a general LC panel, modulation characteristics relative to illumination light obliquely incident on the panel vary in accordance with the incidence angle of the illumination light. Consequently, when the screen is viewed obliquely, the contrast may decrease or the light/dark contrast may be inverted.
In a large-screen display apparatus, the angle between a line normal to the screen and the line of sight is very large at the four corners of the screen, is Even in such a situation, good display characteristics have to be maintained. In addition, in order to enlarge the viewing zone, good display characteristics have to be maintained when the screen is viewed obliquely.
A method of using a backlight which can provide illumination light with good directivity is known as one of methods for improving display characteristics. According to this method, the backlight emits light with very high directivity, e.g. with a broadening angle of .+-.10.degree., and the LC panel is illuminated with this light.
With the light of such a broadening angle, good modulation characteristics can be obtained even in the conventional LC panel. The modulated light is diffused through the diffusion layer 230 provided on the observer side of the LC layer of the LC panel so that the modulated light may broaden over a wide viewing zone. Since the light is diffused after modulation, uniform display characteristics can be obtained over a wide viewing zone if diffusion characteristics are uniform.
As methods of achieving such a parallel-light backlight, two method are proposed, a side-light type method (using a light guide member) and a "just-below-illumination" type method.
In the former method, another optical element for guiding light toward the LC panel 100 is provided on the light-guide plate 240 of the conventional side-light type backlight ("200" in FIG. 1).
On the other hand, in the latter method, an optical device for enhancing directivity of illumination light is provided on a light emission surface side of a conventional just-below type backlight. These methods, however, have the following problems.
In the side-light type method, the use of the light guide plate 240 is indispensable. If the size of the screen is increased, the weight of the LCD apparatus increases because of the light guide plate 240. When the size of the screen is increased, the number of fluorescent tubes needs to be increased in order to increase the flux of light. As a result, the light guide plate 240 with a great thickness is required.
On the other hand, in the just-below type method, the light guide plate is not used and thus the problem of weight is not important. However, in the conventional just-below type method, it is difficult to obtain a uniform illuminance distribution on the display panel 100 and a variance tends to arise in brightness.
For example, even in the case where a reflection plate with a parabolic cylinder shape is used, not only light reflected by the reflection plate but also light directly came from the light source will be made incident on the LC panel.
Thus, such non-uniformity occurs that the screen area close to the light source is bright but the other area on the screen is dark. In the conventional just-below type apparatus, loss of light in the optical element for enhancing directivity becomes relatively high because of its structure. It is therefore difficult to achieve both low power consumption and bright uniform display.
As has been described above, the side-light type method using the light guide plate is an illumination method unsuitable for the large screen. Besides, in the conventional just-below type method, a variance tends to occur in luminance and there is a great transmission loss of illumination light. It is thus difficult to achieve both low power consumption and bright uniform display.